Piezoelectric actuators possess some of the most promising attributes of all developed mechanical actuators. They are capable of operating at frequencies in the MHz range, and with their significant actuation stress, have a maximum power density (W m−3) comparable to hydraulic actuators. Their efficiency ranges from 0.90 to 0.99, well above every other actuator material. The most significant drawback of piezoelectric actuators, however, is the displacement/strain they are capable of producing. With a typical actuation strain of about 0.1%, high strain piezoelectric actuators such as Lead Zirconate Titanate (PZT) stacked to a length of 20 mm will have an unforced displacement of just 20 μm.
Such displacements are largely impractical for broad scale applications, such as robotic systems. Significant research has gone into amplifying the strain that piezoelectric actuators can produce. Means for amplifying the strain produced by piezoelectric actuators include bi-morph and uni-morph bending beam actuators, frequency leveraged “inchworm” actuators, and flextensional strain amplification mechanisms. A conventional flextensional actuator utilizes a rhombus or ellipse-shaped mechanism in which the piezoelectric actuator actively forces the two corners of a major axis causing displacement along a minor axis. Except for the inchworming, or repetitive motion mechanisms, these strain amplification techniques can produce rather limited displacements. Unless multi-stage amplification is used, the output displacement is typically less than 1 mm, which is too short for most robotics applications.
A nonlinearity of structural mechanics, buckling, and singular phenomenon can produce an order-of-magnitude larger effective strain amplification in a single stage. The nonlinearities arising in mechanisms and structural mechanics have typically been thought of as parasitic properties. Strain amplification mechanisms have been designed to keep the output as an approximately linear function of input actuator force and displacement.
In a conventional mechanism, an actuator is a component that produces mechanical work simply by moving a load. The conventional mechanism is merely a uni-directional energy transducer. The conventional mechanisms do not utilize power re-generation and energy harvesting, for example, which are reverse processes transducing mechanical energy back to electric energy. Traditional gear reducers, although optimally tuned to the load, are not necessarily effective for power re-generation and energy harvesting. Impedance matching must be defined differently between forward and backward power transmission. Friction at the gearing and transmission mechanisms often consumes a substantial fraction of available power. The actuator may not be backdriveable.
Backdriveability is an important requirement particularly for a class of machines that physically interact with humans. These include rehabilitation training machines, mobility aids, and power suits. Many of these are creating a growing industry due to demographic changes in modern industrialized countries. Actuators must not only move a human, but also comply with the human and guide the human safely and effectively. The actuators must be bi-directional and interactive to meet these needs.